# Math Help - Geometry of Systems

1. ## Geometry of Systems

Show that all vectors in the vector field F(y,v)=(v,-y) are tangent to circles centered at the origin.

The hint I have is I can use slopes or dot product: I just need some help on how to use them.

2. I'm gonna' give this a whirl alright. So the equation of a circle is:

$x^2+y^2=r^2$

differentiating:

$2x+2yy'=0$

or:

$\frac{dy}{dx}=-\frac{x}{y}$

or parametrically:

$\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=-\frac{x}{y}$

and thus:

\begin{aligned}
\frac{dy}{dt} &=-x \\
\frac{dx}{dt} &=y\end{aligned}

Also, you've got to get Mathematica or something else and just plot these to see globally what's happening. In Mathematica 7, you could use:

StreamPlot[{-x,y},{x,-3,3},{y,-3,3}]

ain't that simple or what?